Dynamic query master agent for query execution

ABSTRACT

A preliminary segment root and a final segment root are selected for each segment. Each time a search query is received, a set of nodes in each segment that will be used to resolve the search query is identified. A preliminary segment root is selected from the set of nodes. Based on statistical data from each node in the set of nodes indicating each node&#39;s capability to act as a final segment root that assembles query-execution data, the preliminary segment root algorithmically selects the final segment root. The other nodes in the set of nodes are notified regarding the identity of the final segment root.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. application Ser. No. 12/951,815 (Attorney Docket Number MFCP.157166), entitled “HYBRID-DISTRIBUTION MODEL FOR SEARCH ENGINE INDEXES,” filed on Nov. 22, 2010, the entirety of which is hereby incorporated by reference.

BACKGROUND

The amount of information and content available on the Internet continues to grow very fast. Given the vast amount of information, search engines have been developed to facilitate searching for electronic documents. In particular, users may search for information and documents by entering search queries comprising one or more terms that may be of interest to the user. After receiving a search query from a user, a search engine identifies documents and/or web pages that are relevant based on the search query. Because of its utility, web searching, that is, the process of finding relevant web pages and documents for user issued search queries has arguably become the one of the most popular service on the Internet today.

Further, search engines typically use a one-step process that utilizes a search index to identify relevant documents to return to a user based on a received search query. Search engine ranking functions, however, have emerged into very complex functions that can be both time consuming and expensive if used for every document that is indexed. Additionally, the storage of data needed for these complex formulas can also present issues, especially when stored in reverse indexes that are typically indexed by words or phrases. The extraction of relevant data needed for the complex formulas, when stored in reverse indexes, is inefficient.

SUMMARY

This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.

Embodiments of the present invention relate to the employment of both atom-sharded and document-sharded distribution across the same set of nodes such that each node, or machine, stores both a portion of a reverse index (e.g., sharded by atom) and a portion of a forward index (e.g., sharded by document). A segment may be assigned a group of documents for which it is responsible. The group of documents is indexed both by atom and document so that there is a reverse index and forward index associated with that group of documents. Each segment comprises multiple nodes, and each node may be assigned a different portion of both the reverse and forward indexes. Further, each node is responsible for performing multiple ranking calculations using both the reverse and forward index portions stored thereon. For instance, a preliminary ranking process may utilize the reverse index and a final ranking process may utilize the forward index. These ranking processes form an overall ranking process that is employed to identify the most relevant documents based on a received search query.

Further embodiments of the present invention are directed to selection of a preliminary segment root and a final segment root. Generally, the preliminary segment root is selected based on any known information at the time it is selected and is only used temporarily until a final segment root is selected. In embodiments, the preliminary segment root utilizes an algorithm to select the final segment root based on statistical data received from various nodes, or machines, that comprise the segment. As will be explained in more detail herein, there are many segments used to resolve a search query, each segment comprising a plurality of nodes, or machines. The preliminary segment root is selected only from those nodes whose search indexes contain a term or atom present in a search query that has been received. That set of nodes includes only the nodes that will be used to execute the particular search query. Once more information can be provided, such as input/output load, current and expected load, including query queue, problem signals associated with the nodes, etc., a final segment root is selected such that the least amount of data is transferred across the network, thus reducing the overall cost of executing a search query.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is described in detail below with reference to the attached drawing figures, wherein:

FIG. 1 is a block diagram of an exemplary computing environment suitable for use in implementing embodiments of the present invention;

FIG. 2 is a block diagram of an exemplary system in which embodiments of the invention may be employed;

FIG. 3 is an exemplary diagram of a hybrid-distribution system, in accordance with embodiments of the present invention;

FIG. 4 is an exemplary diagram of a hybrid-distribution system illustrating payload requirements, in accordance with embodiments of the present invention;

FIG. 5 is a flow diagram showing a method for utilizing a hybrid-distribution system for identifying relevant documents based on a search query, in accordance with embodiments of the present invention;

FIG. 6 is a flow diagram showing a method for generating a hybrid-distribution system for a multi process document retrieval system, in accordance with embodiments of the present invention;

FIG. 7 is a flow diagram showing a method for utilizing a hybrid-distribution system for identifying relevant documents based on a search query, in accordance with embodiments of the present invention; and

FIGS. 8-10 are flow diagrams showing various methods for identifying a segment root from a plurality of nodes, in accordance with embodiments of the present invention.

DETAILED DESCRIPTION

The subject matter of the present invention is described with specificity herein to meet statutory requirements. However, the description itself is not intended to limit the scope of this patent. Rather, the inventors have contemplated that the claimed subject matter might also be embodied in other ways, to include different steps or combinations of steps similar to the ones described in this document, in conjunction with other present or future technologies. Moreover, although the terms “step” and/or “block” may be used herein to connote different elements of methods employed, the terms should not be interpreted as implying any particular order among or between various steps herein disclosed unless and except when the order of individual steps is explicitly described.

As noted above, embodiments of the present invention provide for nodes that form a segment to each store a portion of a reverse index and a forward index for that segment. For instance, out of a total quantity of documents (e.g., one trillion) that are to be indexed, each segment may be allotted a certain portion of documents such that the segment is responsible for indexing and performing ranking calculations for those documents. The portion of the reverse and forward indexes stored on that particular segment is the complete reverse and forward index with respect to the documents assigned to that segment. Each segment is comprised of multiple nodes, which are essentially machines or computational devices with storage capabilities. An independent portion of the reverse index and forward index are assigned to each node in the segment such that each node may be employed to perform various ranking calculations. As such, each node has stored thereon a subset of the segment's reverse index and the forward index, and is responsible for accessing each in various ranking processes within the segment. For instance, an overall ranking process may comprise a matching phase, a preliminary ranking phase, and a final ranking phase. The matching/preliminary phase may require that those nodes whose reverse indexes have indexed a certain atom from a search query be employed to identify a first set of documents that is relevant to the search query. The first set of documents is a set of documents from the documents allocated to the segment. Subsequently, those nodes whose forward indexes have indexed a document identification associated with a document in the first set of documents may be employed to identify a second set of documents that are even more relevant to the search query. The second of documents, in one embodiment, is a subset of the first set of document. This overall process may be employed to limit a set of documents to those that are found to be relevant so that the final ranking process, which is typically more time consuming and costly than the preliminary ranking process, is employed to rank fewer documents than it would be if ranking every document in the index, whether relevant or not.

Accordingly, in one aspect, an embodiment of the present invention is directed to one or more computer-storage media storing computer-useable instructions that, when used by a computing device, cause the computing device to perform a method for utilizing a hybrid-distribution system for identifying relevant documents based on a search query. The method includes allocating a group of documents to a segment, the group of documents being indexed by atom in a reverse index and indexed by document in a forward index and storing a different portion of the reverse index and the forward index on each of a plurality of nodes that form the segment. Further, the method includes accessing the reverse index portion stored on each of a first set of nodes to identify a first set of documents that is relevant to the search query. The method additionally includes, based on document identifications associated with the first set of documents, accessing the forward index portion stored on each of a second set of nodes to limit a quantity of relevant documents in the first set of documents to a second set of documents.

In another embodiment, an aspect of the invention is directed to one or more computer-storage media storing computer-useable instructions that, when used by a computing device, cause the computing device to perform a method for generating a hybrid-distribution system for a multi process document-retrieval system. The method includes receiving an indication of a group of documents assigned to a segment, the segment comprising a plurality of nodes. For the segment, the method further includes indexing the allocated group of documents by atom to generate a reverse index and indexing the allocated group of documents by document to generate a forward index. The method additionally includes assigning a portion of the reverse index and a portion of the forward index to each of a plurality of nodes that form the segment such that each of the plurality of nodes has stored a different portion of the forward index and a different portion of the reverse index.

A further embodiment of the invention is directed to one or more computer-storage media storing computer-useable instructions that, when used by a computing device, cause the computing device to perform a method for utilizing a hybrid-distribution system for identifying relevant documents based on a search query. The method includes receiving a search query, identifying one or more atoms in the search query, and communicating the one or more atoms to a plurality of segments that have each been assigned a group of documents that is indexed both by atom and by document such that a reverse index and a forward index are generated and stored at each of the plurality of segments. Each of the plurality of segments is comprised of a plurality of nodes that are each assigned a portion of the forward index and the reverse index. Based on the one or more atoms, the method identifies a first set of nodes at a first segment whose reverse index portions contain at least one of the one or more atoms from the search query. Additionally, the method includes accessing the reverse index portion stored at each of the first set of nodes to identify a first set of documents that is found to be relevant to the one or more atoms and based on document identifications associated with the first set of documents, identifying a second set of nodes whose forward index portions contain one or more of the document identifications associated with the first set of documents. The method also includes accessing the forward index portion stored at each of the second set of nodes to identify a second set of documents that is a subset of the first set of documents.

In other embodiments of the present invention, a segment root is selected from the nodes that are used to execute a particular query, such as those nodes whose search indexes include a term or atom present in the search index. Initially, a preliminary segment root is selected and is temporarily used to collect data from the other nodes. The preliminary segment root, in one embodiment, collects and aggregates statistical data from the other nodes that will be used to execute the search query. An algorithm is used by the preliminary amendment to determine which node is best suited to act as the final segment root for that particular query. For instance, the segment root with the most data to transfer may be a good selection for the final segment root, as it will not have to transfer a large quantity of data to another node for aggregation. The main concern in selecting a final segment root, in one embodiment, is cost, such as timeliness and ease of transferring data from one node to the final segment root. For example, when a node involved in execution of a particular query identifies documents relevant to the search query, it will have to transfer this data to the final segment root, which aggregates this information from many nodes. The final segment root passes the aggregated query-extraction data from many nodes to another component that assembles similar data from a plurality of segments. As such, the goal is to select a final segment root that will make the overall query-execution process most cost efficient.

As such, one aspect is directed to one or more computer-storage media storing computer-useable instructions that, when used by a computing device, cause the computing device to perform a method for assigning a segment root. The method includes receiving a search query, identifying a set of nodes in a segment that will be used to resolve the search query, and selecting a preliminary segment root from the set of nodes. Further, the method includes receiving, at the preliminary segment root, statistical data from each node in the identified set of nodes, the statistical data indicating each node's capability to act as a final segment root that is responsible for assembling query-execution results from the set of nodes based on the search query. The method additionally includes algorithmically selecting the final segment root from the set of nodes based on the statistical data and notifying the set of nodes about the final segment root so that the nodes know where to send their respective query-execution results.

A second aspect is directed to one or more computer-storage media storing computer-useable instructions that, when used by a computing device, cause the computing device to perform a method for assigning a segment root. The method includes, at a segment comprising a plurality of nodes, receiving a search query to be executed, and identifying a set of nodes from the plurality of nodes that will be used to execute the search query. Prior to execution of the search query, the method includes selecting a preliminary segment root from the plurality of nodes, the selection based on one or more of an expected load for each node or a random selection. Further, the method includes receiving statistical data at the preliminary segment root from each node in the set of nodes that will be used to execute the search query. The statistical data includes current load and cost data associated with sending data across a network. Based on the statistical data, a final segment root is selected that will aggregate query-execution data from the set of nodes during query execution. The search query is then executed.

A third aspect is directed to one or more computer-storage media storing computer-useable instructions that, when used by a computing device, cause the computing device to perform a method for assigning a segment root. The method includes, at a corpus root that comprises a plurality of segments, receiving a search query. Each of the plurality of segments comprises a plurality of nodes. Each node has a portion of a search index stored thereon. The method further includes identifying a set of nodes in each segment that will be used to execute the received search query, and for each of the plurality of segments, identifying a preliminary segment root from the set of nodes. Additionally, statistical data is requested from each node in the set of nodes that will be used to execute the received search query. The statistical data is received from each node of the set of nodes, the statistical data indicating an availability of each node to act as a final segment root that collects query-execution data from the set of nodes in its corresponding segment. The method additionally includes, based on the statistical data, selecting the final segment root for each segment and executing the search query.

Having briefly described an overview of embodiments of the present invention, an exemplary operating environment in which embodiments of the present invention may be implemented is described below in order to provide a general context for various aspects of the present invention. Referring initially to FIG. 1 in particular, an exemplary operating environment for implementing embodiments of the present invention is shown and designated generally as computing device 100. Computing device 100 is but one example of a suitable computing environment and is not intended to suggest any limitation as to the scope of use or functionality of the invention. Neither should the computing device 100 be interpreted as having any dependency or requirement relating to any one or combination of components illustrated.

The invention may be described in the general context of computer code or machine-useable instructions, including computer-executable instructions such as program modules, being executed by a computer or other machine, such as a personal data assistant or other handheld device. Generally, program modules including routines, programs, objects, components, data structures, etc., refer to code that perform particular tasks or implement particular abstract data types. The invention may be practiced in a variety of system configurations, including handheld devices, consumer electronics, general-purpose computers, more specialty computing devices, etc. The invention may also be practiced in distributed computing environments where tasks are performed by remote-processing devices that are linked through a communications network.

With reference to FIG. 1, computing device 100 includes a bus 110 that directly or indirectly couples the following devices: memory 112, one or more processors 114, one or more presentation components 116, input/output (I/O) ports 118, input/output components 120, and an illustrative power supply 122. Bus 110 represents what may be one or more busses (such as an address bus, data bus, or combination thereof). Although the various blocks of FIG. 1 are shown with lines for the sake of clarity, in reality, delineating various components is not so clear, and metaphorically, the lines would more accurately be grey and fuzzy. For example, one may consider a presentation component such as a display device to be an I/O component. Also, processors have memory. The inventors recognize that such is the nature of the art, and reiterate that the diagram of FIG. 1 is merely illustrative of an exemplary computing device that can be used in connection with one or more embodiments of the present invention. Distinction is not made between such categories as “workstation,” “server,” “laptop,” “handheld device,” etc., as all are contemplated within the scope of FIG. 1 and reference to “computing device.”

Computing device 100 typically includes a variety of computer-readable media. Computer-readable media can be any available media that can be accessed by computing device 100 and includes both volatile and nonvolatile media, removable and non-removable media. By way of example, and not limitation, computer-readable media may comprise computer-storage media and communication media. Computer-storage media includes both volatile and nonvolatile, removable and non-removable media implemented in any method or technology for storage of information such as computer-readable instructions, data structures, program modules or other data. Computer-storage media includes, but is not limited to, RAM, ROM, EEPROM, flash memory or other memory technology, CD-ROM, digital versatile disks (DVD) or other optical disk storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other medium which can be used to store the desired information and which can be accessed by computing device 100. Communication media typically embodies computer-readable instructions, data structures, program modules or other data in a modulated data signal such as a carrier wave or other transport mechanism and includes any information delivery media. The term “modulated data signal” means a signal that has one or more of its characteristics set or changed in such a manner as to encode information in the signal. By way of example, and not limitation, communication media includes wired media such as a wired network or direct-wired connection, and wireless media such as acoustic, RF, infrared and other wireless media. Combinations of any of the above should also be included within the scope of computer-readable media.

Memory 112 includes computer-storage media in the form of volatile and/or nonvolatile memory. The memory may be removable, non-removable, or a combination thereof. Exemplary hardware devices include solid-state memory, hard drives, optical-disc drives, etc. Computing device 100 includes one or more processors that read data from various entities such as memory 112 or I/O components 120. Presentation component(s) 116 present data indications to a user or other device. Exemplary presentation components include a display device, speaker, printing component, vibrating component, etc.

I/O ports 118 allow computing device 100 to be logically coupled to other devices including I/O components 120, some of which may be built in. Illustrative components include a microphone, joystick, game pad, satellite dish, scanner, printer, wireless device, etc.

Referring now to FIG. 2, a block diagram is provided illustrating an exemplary system 200 in which embodiments of the present invention may be employed. It should be understood that this and other arrangements described herein are set forth only as examples. Other arrangements and elements (e.g., machines, interfaces, functions, orders, and groupings of functions, etc.) can be used in addition to or instead of those shown, and some elements may be omitted altogether. Further, many of the elements described herein are functional entities that may be implemented as discrete or distributed components or in conjunction with other components, and in any suitable combination and location. Various functions described herein as being performed by one or more entities may be carried out by hardware, firmware, and/or software. For instance, various functions may be carried out by a processor executing instructions stored in memory.

Among other components not shown, the system 200 includes a user device 202, a segment 204, and a hybrid-distribution system server 206. Each of the components shown in FIG. 2 may be any type of computing device, such as computing device 100 described with reference to FIG. 1, for example. The components may communicate with each other via a network 208, which may include, without limitation, one or more local area networks (LANs) and/or wide area networks (WANs). Such networking environments are commonplace in offices, enterprise-wide computer networks, intranets, and the Internet. It should be understood that any number of user devices, segments, and hybrid-distribution system servers may be employed within the system 200 within the scope of the present invention. Each may comprise a single device or multiple devices cooperating in a distributed environment. For instance, the segment may comprise multiple devices arranged in a distributed environment that collectively provide the functionality of the segment 204 described herein. Additionally, other components not shown may also be included within the system 200, while components shown in FIG. 2 may be omitted in some embodiments.

The user device 202 may be any type of computing device owned and/or operated by an end user that can access network 208. For instance, the user device 202 may be a desktop computer, a laptop computer, a tablet computer, a mobile device, or any other device having network access. Generally, an end user may employ the user device 202 to, among other things, access electronic documents by submitting a search query to a search engine. For instance, the end user may employ a web browser on the user device 202 to access and view electronic documents stored in the system.

The segment 204 typically comprises multiple nodes, also termed leaves. In FIG. 2, two nodes are illustrated, including node 1, numbered 210, and node 2, numbered 212. Although two nodes are illustrated in the embodiment of FIG. 2, segments may comprises many more nodes than two (e.g., 10, 40, 100). Two nodes are illustrated for exemplary purposes only. Each segment, such as the segment 204, is allotted a group of documents that it is responsible for. As such, a reverse index and a forward index are generated and stored at the segment 204. While the reverse index and forward index for a particular segment may be generated at the segment 204 itself, in an alternative embodiment, the indexes are generated at some other location or on some other computing device and sent to the segment 204. Further, once the reverse index and the forward index are generated based on the group of documents allotted to the segment 204, both indexes are divided into portions. In one embodiment, the number of portions is equivalent to the number of nodes associated with a particular segment. Therefore, in the case of 40 nodes in a particular segment, both indexes are portioned 40 ways such that each node is responsible for a different portion of each of the reverse index and the forward index. As shown, node 1 has a reverse index portion 214 and a forward index portion 216. Node 2 also has a reverse index portion 218 and a forward index portion 220. While shown separate from the nodes, in one embodiment, the indexes are stored on the node itself. In any case, each node is responsible for a portion of the reverse index and forward index that are indexed based on the group of documents assigned to that segment.

As mentioned, an index may be indexed or sharded by atom or sharded by document. As used herein, sharding refers to the process of indexing a set of documents, whether by atom or by document. There are pros and cons to using each approach separately without the other. For instance, when sharding by document, the pros include the isolation of processing between shards such that only merging of results is needed. Further, per-document information is easily aligned with the matching. Even further, network traffic is small. To the contrary, the cons include that every shard is needed to process any particular query. A minimum of O(KN) disk seeks are needed for a K atom query on N shards if the reverse index data is placed on a disk. When sharding by atom, the pros include the reduced computing such that only K shards are needed to process a K atom query. O(K) disk seeks are required for a K atom query if the reverse index data is placed on a disk. But, to the contrary, the cons include the need for connected processing such that all shards storing atoms which participate in a query need to collaborate. Network traffic is significant, in addition to per-document information not being easily managed. Embodiments of the present invention requires less management of per-document data than traditional approaches. Reasons for this include that some scores are precomputed and scored in the indexes, such as the reverse index, and further refinement and filtering of documents also happens subsequent to the matching phase (L0). As such, the cons described above are greatly reduced with respect to management of per-document data.

Further, each node in a particular segment is capable of performing various functions, including ranking functions that allow for relevant search results to be identified. In some embodiments, the search engine may employ a staged process to select search results for a search query, such as the staged approach described in U.S. Patent Application No. (not yet assigned) (Attorney Docket Number MFCP.157120), entitled “MATCHING FUNNEL FOR LARGE DOCUMENT INDEX.” Here, each node may be capable of employing multiple stages of an overall ranking process. An exemplary ranking process is described below, but is simply one example of ranking processes that may be employed by each node. An overall ranking process may be employed when a search query is received to pair the quantity of matching documents down to a manageable size. When a search query is received, the search query is analyzed to identify atoms. The atoms are then used during the various stages of the overall ranking process. These stages may be referred to as the L0 stage (matching stage) to query the search index and identify an initial set of matching documents that contain the atoms from the search query. This initial process may reduce the number of candidate documents from all documents indexed in the search index to those documents matching the atoms from the search query. For instance, a search engine may search through millions or even trillions of documents to determine those that are most relevant to a particular search query. Once the L0 matching stage is complete, the number of candidate documents is greatly reduced. Many algorithms for locating the most relevant documents, however, are costly and time-consuming. As such, two other stages may be employed, including a preliminary ranking stage and a final ranking stage.

The preliminary ranking stage, also termed the L1 stage, employs a simplified scoring function used to compute a preliminary score or ranking for candidate documents retained from the L0 matching stage described above. The preliminary ranking component 210, as such, is responsible for providing preliminary rankings for each of the candidate documents retained from the L0 matching stage. Alternatively, candidate documents may be scored, and as such given absolute numbers instead of a ranking. The preliminary ranking stage is simplified when compared to the final ranking stage as it employs only a subset of the ranking features used by the final ranking stage. For instance, one or more, but in some embodiments not all, of the ranking features used in the final ranking stage are employed by the preliminary ranking stage. Additionally, features not employed by the final ranking stage may be employed by the preliminary ranking stage. In embodiments of the present invention, the ranking features used by the preliminary ranking stage do not have atom-interdependencies, such as term closeness and term co-occurrence. For example, the ranking features used in the preliminary ranking stage may include, for exemplary purposes only, static features and dynamic atom-isolated components. Static features, generally, are those components that only look into features that are query-independent. Examples of static features include page rank, spam ratings of a particular web page, etc. Dynamic atom-isolated components are components that only look at features that are related to single atoms at a time. Examples may include, for instance, BM25f, frequency of a certain atom in a document, location (context) of the atom in the document (e.g., title, URL, anchor, header, body, traffic, class, attributes), etc.

Once the number of candidate documents has again been reduced by the preliminary ranking stage, the final ranking stage, also termed the L2 stage, ranks the candidate documents provided to it by the preliminary ranking stage. The algorithm used in conjunction with the final ranking stage is a more expensive operation with a larger number of ranking features when compared to the ranking features used in the preliminary ranking stage. The final ranking algorithm, however, is applied to a much smaller number of candidate documents. The final ranking algorithm provides a set of ranked documents, and search results are provided in response to the original search query based on the set of ranked documents. In some embodiments, the final ranking stage as described herein may employ a forward index, as described in U.S. Patent Application No. (not yet assigned) (Attorney Docket Number MFCP.157165), entitled “EFFICIENT FORWARD RANKING IN A SEARCH ENGINE.”

Returning to FIG. 2, the hybrid-distribution system server 206 is comprised of a document allocation component 222, a query parsing component 224, a query distribution component 226, and a result merging component 228. The document allocation component 222 is generally responsible for allocating documents to the various segments used in a given ranking system. For exemplary purposes only, if there are 100 million documents in need of indexing and there are 100 segments available, each segment may be allotted 1 million documents. Or, on a larger scale, if there are 1 trillion documents in need of indexing and there are 100,000 segments available, each segment may be allotted 10 million documents. The documents may be allotted evenly between the segments as demonstrated by the examples above, or may be divided in a different way such that each segment does not have the exact same number of documents for which it is responsible.

When a search query is received via a user interface on the user device 202, for instance, the query parsing component 224 operates to reformulate the query. The query is reformulated from its free text form into a format that facilitates querying the search indexes, such as the reverse indexes and forward indexes, based on how data is indexed in the search indexes. In embodiments, the terms of the search query are parsed and analyzed to identify atoms that may be used to query the search indexes. The atoms may be identified using similar techniques that were used to identify atoms in documents when indexing the documents in the search indexes. For instance, atoms may be identified based on the statistics of terms and query distribution information. The query parsing component 224 may provide a set of conjunction of atoms and cascading variants of these atoms.

An atom, or an atomic unit, as used herein, may refer to a variety of units of a query or a document. These units may include, for example, a term, an n-gram, an n-tuple, a k-near n-tuple, etc. A term maps down to a single symbol or word as defined by the particular tokenizer technology being used. A term, in one embodiment is a single character. In another embodiment, a term is a single word or grouping of words. An n-gram is a sequence of “n” number of consecutive or almost consecutive terms that may be extracted from a document. An n-gram is said to be “tight” if it corresponds to a run of consecutive terms and is “loose” if it contains terms in the order they appear in the document, but the terms are not necessarily consecutive. Loose n-grams are typically used to represent a class of equivalent phrases that differ by insignificant words (e.g., “if it rains I'll get wet” and “if it rains then I'll get wet”). An n-tuple, as used herein, is a set of “n” terms that co-occur (order independent) in a document. Further, a k-near n-tuple, as used herein, refers to a set of “n” terms that co-occur within a window of “k” terms in a document. Thus, an atom is generally defined as a generalization of all of the above. Implementations of embodiments of the present invention may use different varieties of atoms, but as used herein, atoms generally describes each of the above-described varieties.

The query distribution component 226 is essentially responsible for receiving a submitted search query and distributing it amongst the segments. In one embodiment, every search query is distributed to every segment such that each segment provides a preliminary set of search results. For instance, when a segment receives a search query, the segment or a component within the segment determines which nodes will be tasked with performing a preliminary ranking function that utilizes a reverse index portion stored on the nodes. In one case, the selected nodes that are a part of a first set of nodes are those whose reverse index has indexed one or more of the atoms that have been parsed from the search query, as described above. As such, when the search query is reformulated, one or more atoms are identified and sent to each segment. Each of the first set of nodes returns a first set of documents that are found to be relevant to the search query based on a preliminary ranking function, as briefly described above. Subsequently, a second set of nodes is determined. In one embodiment, each of these nodes has stored in its respective forward index at least one of the documents in the first set of documents. Each of the second set of nodes performs a final ranking function using forward index data and other considerations and as a result, a second set of documents is identified. In one embodiment, each of the documents in the second set is included in the first set, as the document identifications associated with the first set of documents are used in the final ranking stage.

The result merging component 228 is given the search results (e.g., document identifications and snippets) from each segment and from those results, a merged and final search results list is formed. There are various ways that the final search results list is formed, including simply removing any duplicate documents and putting each document in a list in an order determined by the final rankings. In one embodiment, a component similar to the result merging component 228 is present on each segment such that the results produced by each node are merged into a single list at the segment before that list is sent to the result merging component 228.

Turning now to FIG. 3, an exemplary diagram is shown of a hybrid-distribution system 300, in accordance with embodiments of the present invention. FIG. 3 illustrates various components, including a corpus manager 310, a corpus root 312, and two segments, segment 314 and segment 316. More than two segments may be provided, as indicated by ellipsis 318. The corpus manager 310 holds the state of which processes serve which shard of the forward index and reverse index. It further holds the temperature and state of each of the processes. This data is used to generate the set of processes to use for federating the query out to the different segments. The corpus root 312 is a top-level root process that also performs query planning functions. The corpus root 312 will scatter the query across all the required segments and gather and merge results, and may include custom logic. Each segment has a segment root, such as segment root 320 and segment root 322. A segment root serves as process for federation of queries and aggregation of results from the processed being federated. The segment root 322 is likely to be a dynamic process reassigned to the leaf or node most optimal for final query assembly.

As shown, each segment root is comprised of multiple nodes. Because of space constraints, three nodes are illustrated for segment root 320 and segment root 332. Segment root 320 is comprised of node 322, node 324, and node 326. Ellipses 328 indicates that more than three nodes are contemplated to be within the scope of the present invention. Segment root 332 comprises node 334, node 336, and node 338. As any number of nodes may comprise a segment root, ellipses 340 indicates any additional quantity of nodes. As mentioned, each node is a machine or computational device that is capable of performing multiple calculations, such as ranking functions. For instance, in one embodiment, each node comprises an L01 matcher 322A and an L2 ranker 322B, as shown at node 322. Similarly, node 334 comprises an L01 matcher 334A and an L2 ranker 334B. These are described in more detail above, but the L0 matching and L1 ranking phases (preliminary ranking phase) of an overall ranking process may be combined and collectively called the L01 matcher. As each of the nodes comprises an L01 matcher and an L2 ranker, each node must also have stored a portion of a reverse index and forward index, as the L01 matcher, in one embodiment, utilizes a reverse index and the L2 ranker utilizes a forward index. As mentioned, each of the nodes may be assigned a portion of the reverse and forward indexes that belong to the segment. The segment communication bus 330 associated with segment 314 and the segment communication bus 342 associated with segment 316 allow for each of the nodes to communicate when necessary, such as with the segment root.

FIG. 4 is an exemplary diagram of a hybrid-distribution system 400 illustrating payload requirements, in accordance with embodiments of the present invention. System 400 is an illustration of a single segment root 410 having multiple nodes. Here, six nodes are illustrated (including nodes numbered 412, 414, 416, 418, 420, and 422). Although six nodes are illustrated, any number may be utilized to carry out embodiments of the present invention. As previously indicated, each node has the functionality to perform various ranking calculations, including those in a matching stage (L0), a preliminary ranking stage (L1), and a final ranking stage (L2). As such, node 412, for example, has both an L01 matcher 412A for the L0 and L1 stages described herein and an L2 ranker 412B for the L2 stage described herein. The payload for the different stages, however, may greatly differ. To better illustrate this, the payload for the L01 matcher is shown in a first pattern, as indicated by numeral 424, and the payload for the L2 matcher is shown in a second pattern, as indicated numeral 426.

The group of documents that is allocated to a particular segment is indexed or sharded both by atom (reverse index) and by document (forward index). These indexes are divided into portions equal to the number of nodes that comprise that particular segment. In one embodiment, there are forty nodes, and thus each of the reverse index and the forward index is divided into forty portions and stored at each of the respective nodes. When a search query is submitted to a search engine, the query is sent to each segment. It is the segment's responsibility to identify a first set of nodes whose reverse index has one or more of the atoms from the query indexed. Using this method, if the query is parsed into two atoms, for instance “William” and “Shakespeare” from the query “William Shakespeare,” the most number of nodes in a segment that would be engaged for the L01 matcher would be two. This is shown in FIG. 4, as the L01 matcher associated with node 412 and node 416 are the only two that are identified for use in the L01 matching process. Since the documents assigned to each segment are indexed by atom, the each atom is indexed only once in the reverse index such that any particular atom is present in only one of the reverse index portions allocated to the nodes of that segment. In an exemplary scenario, once the first set of nodes is identified, the atom from the search query that matches the atom in the reverse index is sent to the appropriate node. That node performs many computations such that a set of documents is identified. This first set of documents, in one embodiment, contains those documents that have received the highest ranking from the preliminary ranking phase.

This first set of documents is collected at the segment root 410 from each of the nodes from the first set of nodes, including nodes 412 and 416. These results are combined in any of a number of ways so that the segment root 410 can next identify a second set of nodes that will be used in conjunction with the final ranking stage. As shown, each L2 ranker is employed in the final ranking stage, or the L2 stage. This is because each node has stored a portion of the forward index for that segment, and as such there is a good chance that most or all forward indexes will need to be accessed in the final stage of ranking. In the final ranking stage, each node in the second set of nodes is given the document identification that it contains in its forward index so that the node can rank that document based on, at least, data found in the forward index. Because most or all of the nodes are employed in the final ranking stage, as shown in the system 400 of FIG. 4, the payload for the final ranking stage is typically greater than the payload for the matching/preliminary ranking stage. The segment communication bus 428 allows for the nodes to communicate with other components such as, for example, the segment root 410.

Referring to FIG. 5, a flow diagram illustrates a method 500 for utilizing a hybrid-distribution system for identifying relevant documents based on a search query, in accordance with embodiments of the present invention. Initially, a group of documents is allocated to a segment at step 510. The group of documents, either prior to or subsequent to being received at the segment, is indexed both by atom in a reverse index and by document in a forward index, indicated by step 512. As such, the documents indexed in the forward index are the documents that comprise the group of documents allocated to that segment and the atoms in the reverse index are parsed from the content of these documents. At step 514, a portion of the reverse index and the forward index is stored at each node in the segment. Generally segments are comprised of a plurality of nodes. Each node is a machine or computation device that is capable of performing ranking computations based on the reverse index portion and forward index portion stored thereon. In one embodiment, each node stores a different or unique portion of the segment's reverse index and forward index.

Step 516 indicates that the reverse index portion is accessed at each node of a first set of nodes. Each node in the first set of nodes has been identified as having indexed one of the atoms of a received search query. A first set of documents is identified at step 518. These documents, in one embodiment, have been ranked using a preliminary ranking function so that the most relevant documents can be identified. This step may correspond, for instance, to the L1 preliminary ranking phase and/or the L0 matching phase. Based on document identifications associated with the documents in the first set of documents, the forward index portion is accessed at each of a second set of nodes, shown at step 520. This step may correspond to the L2 final ranking stage. This effectively limits the quantity of relevant documents for a particular search query. As such, the quantity of documents is limited to a second set of documents, shown at step 522. In many or most instances, the number of nodes in the second set is greater than the nodes in the first set, as described in greater detail above. This is because a search query may have only two atoms such that two nodes, at the most, are needed for the L01 matching phase, but thousands of documents are identified as being relevant to the two atoms of the search query, and as such many more nodes may be employed to use their respective forward indexes to perform the final ranking computations to identify the second set of documents. Further, in embodiments, because the final ranking function utilizes the document identifications produced from the preliminary ranking function, the number of document in the second set is less than the number of documents in the first set such that each document in the second set is also contained in the first set.

In one embodiment, the overall process may involve receiving a search query. One or more atoms in the search query are identified, and once each segment is aware of the one or more atoms, a first set of nodes is identified in the segment that contains at least one of the one or more atoms from the search query. Each of the nodes in the first set of nodes sends a first set of documents (e.g., document identifications) to the segment root, for example, so that the segment root can consolidate (e.g., delete duplicates) and merge the results. A second set of nodes then sends the segment root a second set of document. Similarly, the segment root consolidates and merges the results to generate a final set of documents that is presented to the user in response to the search query.

Turning to FIG. 6, a flow diagram is illustrated of a method 600 for generating a hybrid-distribution system for a multiprocess document retrieval system, in accordance with embodiments of the present invention. At step 610, an indication of a group of documents is received, the group of documents being assigned to a segment that receives the group of documents. The segment comprises multiple nodes (e.g., ten, forty, fifty). The group of documents is indexed by atom to generate a reverse index, shown at step 612. At step 614, the group of document is indexed by document to generate a forward index. At step 616, a portion of the reverse index and a portion of the forward index is assigned to each node that comprises the segment. In embodiments, each node is assigned a different portion of the reverse and forward indexes such that a particular atom is indexed in only one node's forward index within a segment.

In embodiments, at the segment, an indication is received of one or more atoms that have been identified from a search query. A first set of nodes is identified whose reverse index portions include at least one of the one or more atoms. These nodes are each capable of performing various ranking functions. A first set of documents is identified based on the reverse index portions of the first set of nodes. Each node in the first set may produce a first set and send it to the segment root such that the various first sets of nodes can be consolidated and merged. In one instance, the first set of documents is produced by way of a preliminary ranking process of a multistage ranking process that utilizes the reverse index portions stored thereon. Further, a second set of nodes may then be identified whose forward index portions have indexed one or more document identifications corresponding to the first set of documents. A second set of documents may then be identified based, partially, on data stored in the forward index, and may compute features in real-time rather than using precomputed scores. The second set of documents may be identified based on a final ranking process of a multistage ranking process that utilizes the forward index. Once the second set of documents from each node in the second set of nodes is consolidated and merged, it is also merged with the second sets of documents from all of the other segments so that a final set of documents is formed and returned to the user as search results.

FIG. 7 is a flow diagram showing a method 700 for utilizing a hybrid-distribution system for identifying relevant documents based on a search query, in accordance with embodiments of the present invention. Initially, at step 710, a search query is received. In one embodiment, the query is augmented or altered, such as using a spell correction tool or stemming. Atoms in the search query are identified at step 712. At step 714, the atoms are communicated to various segments. Each segment has been assigned a group of documents, the group of documents being indexed both by atom and by document to from a reverse index and a forward index stored at each segment. Each segment is comprised of a plurality of nodes that are each assigned a portion of the forward index and the reverse index. At step 716, a first set of nodes is identified whose reverse index portions contain at least one of the atoms from the search query. At step 718, the reverse index portions of each of the nodes in the first set of nodes is accessed to identify a first set of relevant documents. Based on document identification associated with each document in the first set of documents, a second set of nodes is identified at step 720. Each node in the second set of nodes has stored in its respective forward index portion at least one of the document identifications such that the node can perform a ranking process on each document. The forward index portion at each node in the second set of n odes is accessed at step 722 to limit the number of relevant documents. In one embodiment, each document in the second set of documents is also contained in the first set of documents. Based on the second set of documents, search results are generated (e.g., by compiling the second sets of documents from the plurality of segments) and presented to the user.

Referring now to FIG. 8, a method 800 is provided for assigning a segment root for each segment of a corpus root. In embodiments, such as that described below, a segment root may take the form of a preliminary segment root and final segment root. For instance, a preliminary segment root is selected based on information known at the time, such as the current or expected load of each node in a segment, or may even be selected at random, such as on a rotating schedule. The final segment root is selected based on many factors and will be discussed in more detail below. Generally, a segment root serves as a process for federation of queries and aggregation of results from the process being federated. The process of selecting a preliminary and final segment root, such as segment root 322 shown in FIG. 3, is likely to be a dynamic process. For instance, the process of assigning a preliminary and final segment root may take place for each received query and across multiple segments simultaneously. The node chosen for the final segment root is thought to be optimal for final query assembly.

As shown in FIG. 3 herein, each segment root is comprised of multiple nodes. Because of space constraints, three nodes are illustrated for segment root 320 and segment root 332. Segment root 320 is comprised of node 322, node 324, and node 326. Ellipses 328 indicates that more than three nodes are contemplated to be within the scope of the present invention. Segment root 332 comprises node 334, node 336, and node 338. As any number of nodes may comprise a segment root, ellipses 340 indicates any additional quantity of nodes. As mentioned, each node is a machine or computational device that is capable of performing multiple calculations, such as ranking functions. For instance, in one embodiment, each node comprises an L01 matcher 322A and an L2 ranker 322B, as shown at node 322. Similarly, node 334 comprises an L01 matcher 334A and an L2 ranker 334B. These are described in more detail above, but the L0 matching and L1 ranking phases (preliminary ranking phase) of an overall ranking process may be combined and collectively called the L01 matcher. As each of the nodes comprises an L01 matcher and an L2 ranker, each node must also have stored a portion of a reverse index and forward index, as the L01 matcher, in one embodiment, utilizes a reverse index and the L2 ranker utilizes a forward index. As mentioned, each of the nodes may be assigned a portion of the reverse and forward indexes that belong to the segment. The segment communication bus 330 associated with segment 314 and the segment communication bus 342 associated with segment 316 allow for each of the nodes to communicate when necessary, such as with the segment root.

Returning to FIG. 8, a search query is initially received at step 810. Referring to FIG, 3, the search query may be received at the corpus root 312, which then distributes the search query or portions thereof to the segments. At step 812, a set of nodes in a segment is identified based on whether each node will be used to resolve the search query. As mentioned, each node is assigned a group of documents from which a reverse index and a forward index is generated. As such, depending on the atoms in the search query, some nodes will be used for a particular query and some nodes will not. The set of nodes identified at step 812 is identified based on the nodes that will be used to resolve a particular query, or the hash of the atoms in the query. A hash function takes a term or atom in the search query and determines which nodes have stored thereon the posting list with that particular term or atom. This allows for an identification of the nodes that will be used to resolve that particular search query. A posting list is simply a list of terms and those documents that contain that term. An algorithm may be used for the hash function. Exemplary algorithms include MD5 or CRC, but others are certainly contemplated to be within the scope of the present invention.

At step 814, a preliminary segment root is selected from the set of nodes. In one embodiment, the preliminary segment root is selected at random, such as on a rotating basis. In another embodiment, however, the preliminary segment root is selected based on an expected load such that the node having the lowest expected load is selected as the preliminary segment root. In one instance, this may be the node with the lowest number of recorded outstanding requests, such as a current load such that the node having the lowest current load of outstanding queries is selected as the preliminary segment root. Once a preliminary segment root is selected from the set of nodes, statistical data from each node in the set of nodes is received at the preliminary segment root, shown at step 816. In one embodiment, the preliminary segment root requests that this data be sent by way of, for example, a communication bus connected to each of the nodes. Statistical data may indicate each node's capability to act as a final segment root. The final segment root is responsible for assembling query-execution results from the set of nodes based on the search query. Statistical data may include, for exemplary purposes only, a length of each node's posting list, an input/output load, problem signals associated with a particular node, or a quantity of data that would be required to be transferred to the final segment root. Generally, the node that is thought to incur the least cost (e.g., time, monetary) when acting as the final segment root is chosen.

For instance, if a particular node has an extremely long posting list, a large amount of data would need to be transferred across the network to the final segment root so that the final segment root can aggregate the query-extraction data from all of the nodes. In one embodiment, this particular node having a large amount of data to be transferred may be selected as the final segment root so that its data does not have to be sent to another node, which would cause a high-cost transfer of data. As previously mentioned, a node that is sending out problem signals may have some issues or problems. Many types of auxiliary signals can indicate that the node's performance of getting data out is impaired. As also mentioned, input/output load may be taken into account when selecting the final segment root. For instance, this may encompass the length of a node's queue to extract data from a hard disk. Further, if there are three nodes whose posting lists include the word “dog,” when a query is received that also includes the word “dog,” the node having the lowest load may be selected, as it would have more time to act as the final segment root. Other factors including bandwidth may also be included when the final segment root is determined. In one instance, the preliminary segment root actually executes an algorithm that determines the final segment root.

At step 818, the final segment root is algorithmically selected from the set of nodes based on the statistical data. In some embodiments, the preliminary segment root and the final segment root are the exact same node, but in other embodiments, they are different nodes. An algorithm may be used to make the determination as to the node that is to be used as the final segment root for a particular query. The query takes into account the statistical data described above. At step 820, the set of nodes is notified about the identify of the final segment root so that the nodes know where to send their respective query-execution results. In one embodiment, the preliminary segment root takes it upon itself to transfer itself to the final segment root, or if it is selected as the final segment root, the preliminary segment root may communicate to the other nodes that it is now the final segment root.

In embodiments, the search query is executed using the set of nodes previously identified. As mentioned herein, a first set of nodes may be identified as taking part in the preliminary ranking stage (e.g., utilizing the reverse indexes stored on the nodes) and a second set of nodes may be identified as taking part in the final ranking stage (e.g., utilizing the forward indexes stored on the nodes). The final segment root may be used to collect and aggregate data from both the preliminary and the final ranking stages. For instance, the nodes involved in the preliminary ranking stage return listings of documents that contain a certain term or atom in the search query. The nodes involved in the final ranking stage return documents, such as document identification, that are most relevant to the search query. As such, query-execution results may refer to either the results from the preliminary ranking stage, a final ranking stage, or both. Even further, a multiple step ranking process may not be used. In instances where there is a single ranking or search process, that single set of results is collected and aggregated by the final segment root.

The method described in FIG. 8 may be utilized as a system. For instance, various system components could be used to select a preliminary and final segment root. These components may include, for exemplary purposes only, a preliminary segment root selection component, a statistical data receiving component, a final segment root selection component, and a query execution component. These components may communicate with one another over a network, such as network 208 illustrated in FIG. 2. The preliminary segment root selection component is responsible for gathering data and performing a hash calculation to determine which nodes in that particular segment will be used to execute the search query. As mentioned, each node has a portion of a search index and a posting list. The posting list includes atoms and documents in which that atom is present. The statistical data receiving component may request and receive from the nodes statistical data that indicates the availability and ability of each node to act as the final segment root. The final segment root selecting component is responsible for selecting the final segment root, in accordance with the statistical data received from the nodes that will be used to execute the search query. In one embodiment, the preliminary segment root uses an algorithm to make this determination. Lastly, the query execution component distributes the search query or portions thereof to the final segment roots, which then distributes portions of the search query to the appropriate nodes in that segment. The nodes determine which documents are most relevant to the query or the portion of the query, and communicate that data to the final segment root by way of, for example, a communication bus. As mentioned, this process simultaneously occurs for multiple segments.

Turning to FIG. 9, a method 900 is illustrated for selecting a segment root from a plurality of nodes. Initially, at step 910, a search query is received at a segment that comprises a plurality of nodes. As mentioned, there are multiple segments (e.g., hundreds of segments) that each execute a search query simultaneously. Each segment comprises a plurality of nodes that are each assigned a portion of the documents that are indexed in one or more search indexes. As such, each node has stored a portion of a reverse and a forward index that are indexed by atom and document, respectively. At step 912, a set of nodes from the plurality of nodes are identified as being used to execute the received search query. A hash function, for instance, may be used to make this determination as to which nodes will be used. This may depend on the index stored on each node, such that those nodes whose indexes have stored a particular term or atom in the search query are identified as being used to resolve that particular search query. At step 914, prior to execution of the search query, a preliminary segment root is selected from the plurality of nodes. The selection of the preliminary segment root is based on an expected load for each node, a current load for each node, random selection, etc. The information available at the time of the preliminary segment root selection is used to make the selection. The preliminary segment root functions as a preliminary segment root until the final segment root is selected.

Statistical data is received at the selected preliminary segment root at step 916. The statistical data, prior to being received, may be requested from each node in the set of nodes used to execute the search query and is also received at the preliminary segment root. As previously described, statistical data may include one or more of the length of each node's posting list or other data that would have to travel across the network, the input/output load on the nodes, such as how long the queue length is to extract data from the hard disk, problem signals associated with the nodes, cost, etc. At step 918, a final segment root is selected based on the received statistical data. The final segment root collects and aggregates query-execution data from the set of nodes during query execution. The final segment root, in one embodiment, accepts the search query from an external source, such as a server. The final segment root may also or alternatively accept the search query from a corpus root, such as corpus root 312 shown in FIG. 3. At step 920, the search query is executed. As mentioned, there may be one or more phases of query execution, such as a preliminary ranking stage and a final ranking stage.

FIG. 10 illustrates a method 1000 for selecting a segment root. At a corpus root having one or more segments, a search query is received at step 1010. Each of the segments in the corpus root have a plurality of nodes, each having a portion of a search index stored thereon. For instance, each node may have stored thereon a portion of a reverse index organized by atoms and a portion of a forward index organized by document. This may be the case when multiple ranking or search stages are used to provide search results based on a search query. At step 1012, a set of nodes is identified in each segment that will be used to execute the received search query. Multiple segments carry out the process of selecting a preliminary and final segment root simultaneously. Further, this process may occur for each and every search query received at the corpus root. At step 1014, for each of the segments in the corpus root, a preliminary segment root is identified from the set of nodes. Statistical data is requested at step 1016, such as by the preliminary segment root, from each of the nodes in the set of nodes that will be used to execute the received search query. Statistical data includes any data that indicates an ability or availability of a node to be selected as the final segment root. The overall goal is to transfer as little data as possible over the network, such as from the nodes to the final segment root. The most cost-efficient node is thus selected as the final segment root. In some instances, the overall goal may be geared more toward load, but in other instances, the preliminary segment root may weigh heavier in favor of network capacity.

At step 1018, statistical data is received at the preliminary segment root from each node in the set of nodes. As mentioned, the statistical data indicates an availability of each node to act as a final segment root that collects query-execution data from the set of nodes in its corresponding segment. In one embodiment, a communication bus is used to communicate the statistical data and other data from the nodes to the preliminary or final segment root. A final segment root is selected for each segment at step 1020 based on the statistical data. At step 1022, the search query is executed. In some embodiments, prior to the execution of the search query, a finalization message is communicated to the plurality of nodes, or at least the set of nodes indicating an identification of the final segment root so that the nodes know where to send their respective data, including query-execution data. The final segment root, after the query is executed, receives the query-execution data from each node in the set of nodes.

The present invention has been described in relation to particular embodiments, which are intended in all respects to be illustrative rather than restrictive. Alternative embodiments will become apparent to those of ordinary skill in the art to which the present invention pertains without departing from its scope.

From the foregoing, it will be seen that this invention is one well adapted to attain all the ends and objects set forth above, together with other advantages which are obvious and inherent to the system and method. It will be understood that certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations. This is contemplated by and is within the scope of the claims. 

1. One or more computer-storage media storing computer-useable instructions that, when used by a computing device, cause the computing device to perform a method for assigning a segment root, the method comprising: receiving a search query; identifying a set of nodes in a segment that will be used to resolve the search query; selecting a preliminary segment root from the set of nodes; receiving, at the preliminary segment root, statistical data from each node in the identified set of nodes, the statistical data indicating each node's capability to act as a final segment root that is responsible for assembling query-execution results from the set of nodes based on the search query; algorithmically selecting the final segment root from the set of nodes based on the statistical data; and notifying the set of nodes about the final segment root so that the nodes know where to send their respective query-execution results.
 2. The computer-storage media of claim 1, wherein each node in the set of nodes has stored thereon a portion of a search index that is used to execute the search query.
 3. The computer-storage media of claim 1, wherein the preliminary segment root is selected at random.
 4. The computer-storage media of claim 1, wherein the preliminary segment root is selected based on an expected load such that the node having the lowest expected load is selected as the preliminary segment root.
 5. The computer-storage media of claim 1, wherein the preliminary segment root is selected based on a current load such that the node having the lowest current load of outstanding queries is selected as the preliminary segment root.
 6. The computer-storage media of claim 1, wherein the preliminary segment root and the final segment root are selected for each segment in response to the received search query.
 7. The computer-storage media of claim 1, wherein the statistical data comprises one or more of a length of each node's posting list, an input/output load, problem signals associated with a particular node, or a quantity of data that would be required to be transferred to the final segment root.
 8. The computer-storage media of claim 1, further comprising executing the search query using the set of nodes.
 9. The computer-storage media of claim 8, wherein the query-execution results from multiple segments are aggregated and returned to a user.
 10. One or more computer-storage media storing computer-useable instructions that, when used by a computing device, cause the computing device to perform a method for assigning a segment root, the method comprising: at a segment comprising a plurality of nodes, receiving a search query to be executed; identifying a set of nodes from the plurality of nodes that will be used to execute the search query; prior to execution of the search query, selecting a preliminary segment root from the plurality of nodes, the selection based on one or more of an expected load for each node or a random selection; receiving statistical data at the preliminary segment root from each node in the set of nodes that will be used to execute the search query, wherein the statistical data includes current load and cost data associated with sending data across a network; based on the statistical data, selecting a final segment root that will aggregate query-execution data from the set of nodes during query execution; and executing the search query.
 11. The method of claim 10, wherein each node in the plurality of nodes has stored thereon a portion of a search index used to identify relevant documents based on the search query.
 12. The method of claim 10, wherein the final segment root is selected based on a lowest load and lowest cost of the set of nodes.
 13. The method of claim 10, wherein the final segment root accepts the search query from an external source.
 14. The method of claim 10, wherein each of the plurality of nodes has stored thereon a portion of a reverse index that is indexed by atom and a portion of a forward index that is indexed by document.
 15. The method of claim 14, wherein executing the search query further comprises a preliminary ranking process that utilizes the set of nodes whose reverse indexes include an atom contained in the search query.
 16. The method of claim 15, wherein executing the search query further comprises a final ranking process that utilizes a different set of nodes whose forward indexes include a document identified from the preliminary ranking process.
 17. One or more computer-storage media storing computer-useable instructions that, when used by a computing device, cause the computing device to perform a method for assigning a segment root, the method comprising: at a corpus root that comprises a plurality of segments, receiving a search query, wherein each of the plurality of segments comprises a plurality of nodes that each has a portion of a search index stored thereon; identifying a set of nodes in each segment that will be used to execute the received search query; for each of the plurality of segments, identifying a preliminary segment root from the set of nodes; requesting statistical data from each node in the set of nodes that will be used to execute the received search query; receiving the statistical data from each node of the set of nodes, the statistical data indicating an availability of each node to act as a final segment root that collects query-execution data from the set of nodes in its corresponding segment; based on the statistical data, selecting the final segment root for each segment; and executing the search query.
 18. The computer-storage media of claim 17, wherein the statistical data is communicated to the preliminary segment root by way of a communication bus accessible by each of the plurality of nodes.
 19. The computer-storage media of claim 17, further comprising communicating a finalization message to the plurality of nodes indicating an identification of the final segment root.
 20. The computer-storage media of claim 17, further comprising, at each final segment root, receiving the query-execution data from the set of nodes. 